Frequently, servo motor position control is performed by a servo system having a cascade configuration in which a position control loop is set to a main loop while a minor loop is formed in the order of a velocity control loop and a current control loop. In consideration of stability and convergence of the servo system, the position control is performed by proportional control, and the velocity control and the current control are performed by proportional-integral control.
In the servo motor position control, a driving position (hereinafter also simply referred to as “position”) of the servo motor is obtained by counting the number of pulses output from a pulse encoder using a pulse encoder attached to the servo motor. Generally, a driving velocity (hereinafter also simply referred to as “velocity”) of the servo motor is obtained by dividing the number of pulse encoder pulses generated within a constant sampling time by the sampling time.
At this point, when the servo motor reaches the target position and stops, the pulse encoder does not output the pulse any more, and the position control is not performed. Therefore, depending on a control method, the servo motor may oscillate in a range of minimum resolution of the pulse encoder to generate hunting.
That is, the state in which the servo motor is stopped means that the position of the servo motor is located within a predetermined accuracy range from an instruction position, and it can also be said that the state is a stop instruction state. Methods, such as a method for decreasing a gain of a velocity control loop and a method for switching an electric phase angle of the servo motor from 90 degrees to zero degree when a position deviation between an actual position of the servo motor and an instruction position is zero or close to zero, are proposed as the method for controlling the hunting in the stop instruction state, which is caused by the position and velocity detection system using the pulse encoder.    Patent Document 1: Japanese Patent Application Laid-Open No. 62-245312    Patent Document 2: Japanese Patent Application Laid-Open No. 07-123767    Patent Document 3: Japanese Patent Application Laid-Open No. 11-332278